Because of cost reduction and added safety, bottom loading has been used for many years on highway transportation tanks. At first, the cut-off controls were directed to cutting off the flow when the liquid level in the tank reached a predetermined level. Volume measuring devices were commonly used, but no satisfactory system evolved.
The laws requiring the recovery of vapors have given impetus to bottom loading, but now most terminals load by preset meters. There still remains the chance, (1) that the meter could be incorrectly set, (2) the compartments would not be completely drained, or (3) connections would be mis-mated. Any one of these three conditions could result in spills. To guard against this, an override system actuated by the liquid level is mandatory.
Many override systems have been devised, including float switches, float valves, fiber optics, thermistors, electric eyes, liquid jets and many more. Some tanks carry two systems in order to be loaded at different terminals.
Most systems are expensive and many require electrical connections to the bulk plant. Few are interchangeable. Also, many of them are wrecked by vibration, particularly the float valves.